The present invention generally relates to the formation of materials useful for electronic applications, in particular for photovoltaic solar cells.
Manufacturers of electronic devices, in particular photoelectronic devices and more specifically photovoltaic devices, are increasingly employing non-elemental materials such as III-V, II-VI and I-III-VI semiconductors and alloys, mixtures and layered structures of such materials. The constituent layers in such photoelectronic devices are typically fabricated using vapor phase deposition processes such as vacuum evaporation, sputtering and chemical vapor deposition. Vapor phase processes are useful for small-scale research and for high-precision processing of high-value, small-area devices such as integrated circuits. Vapor phase processes have yielded solar cells with high sunlight-to-electricity conversion efficiencies; but it is difficult to deposit uniform films on large areas using coincident vapor phase processes in which the constituent elements are co-deposited, hence coincident vapor phase processes are typically costly to scale up to large device sizes with the control and through-put required for commercial production.
Various researchers have explored sequential vapor phase processes. CuInSe2 films are typically formed by sequentially depositing solid layers of Cu and In elemental metals and subsequently reacting the Cuxe2x80x94In composite layers with a source of Se to form CuInSe2. By solid layer we mean a substantially solid layer of material with minimal included void space. Multi-step, sequential processes substitute separate sequential deposition of constituents in place of co-deposition of constituents, with the intent of mitigating materials interactions that typically complicate co-deposition processes. However, this separation into sequential depositions of different constituents can introduce new complications; for example, vapor phase deposited indium tends to de-wet, forming localized islands, and solid layers of Cuxe2x80x94In alloys can segregate into In-rich and Cu-rich phases when heated, with the result that extreme care is required to maintain the desired planar Cuxe2x80x94In layer structure and lateral composition uniformity during the early stages of subsequent reactions to form CuInSe2.
Various researchers have explored techniques for stabilizing metal precursor layers such as Cuxe2x80x94In for CuInSe2 against de-wetting and phase segregation. For example, thin layers of a chalcogenide metal such as tellurium can be deposited on the substrate prior to the deposition of Cu and In in order to form telluride compounds that mitigate indium de-wetting and phase segregation, and solid layers of binary chalcogenide compounds such as Cu2Se and In2Se3 can be deposited and subsequently interdiffused to form ternary CuInSe2. Such processes use chalcogenide compounds to stabilize the primary non-chalcogen constituent metals Cu and In against phase segregation during deposition and subsequent processing.
Other researchers have explored vapor phase processing of oxide-containing phase-stabilized precursors and mixtures of such precursors with elemental metals and non-oxide chalcogenides. For example, chalcogenide solid films can be formed by depositing a metal oxide solid film and annealing the oxide film in a gas or vapor containing a metal chalcogen such as S, Se, Te or a mixture thereof. Such processes can also utilize layers of single-phase oxide particulates, such as Cu2In2O5 particles or such as Cu2O particles mixed with In2O3 particles, and can also utilize solid layers of a mixture of metal and oxide, selenide or sulfide compound constituents. While mitigating certain complications of sequential processes, these phase-stabilized precursor improvements leave unresolved the inherent complexities of achieving the constituent layer uniformity necessary to achieve high-quality semiconductor films using vapor phase processes.
Various researchers have explored alternatives to vapor phase processes for fabricating semiconductor materials for various photoelectronic applications. Electrodeposition can be used to sequentially deposit solid layers of constituent metals such as Cu and In that are subsequently reacted with chalcogenide metals such as Se to form compound semiconductor films. The chalcogenide metal can be embedded in the electroplated solid metal layers by adding Se particles to the electroplating bath so as to incorporate Se particles into one or more of the metallic layers. Additional Se can be added by screen printing a solid Se layer on the Cuxe2x80x94Inxe2x80x94Se precursor layers, or by annealing the electrodeposited layers in Se vapor. While avoiding some of the disadvantages of vapor phase processes, such electrodeposition processes are plagued with the challenges of uniform high-rate electrodeposition on large-area substrates and introduce electrodeposition-specific complications such as metal-contaminated waste treatment, recovery and disposal.
Other researchers have explored alternatives to both vapor phase and electrodeposition processes. For example, spray pyrolysis techniques can be used to deposit metal oxide solid layers, and the oxide layers can subsequently be annealed in chalcogenide metal vapor to convert the oxide layers to chalcogenide films. Spray pyrolysis is convenient for depositing multi-component oxides on large areas; but the materials use efficiencies of spray pyrolysis processes are generally low, hence manufacturing materials costs are generally high.
Alternatively, one can form precursor layers by screen printing a paste of particles or by painting a substrate with a slurry of particles. For example, one can form a Cuxe2x80x94Inxe2x80x94Se powder, prepare a paste from the powder, screen print layers of the paste, and anneal the layers to form CuInSe2 films. Cuxe2x80x94Inxe2x80x94Se powders prepared by ball milling or grinding reportedly yield median particle diameters of 1.5 xcexcm and larger. Median powder particle diameter determines minimum pinhole-free CuInSe2 layer thickness; particle diameters of 1.5-2 xcexcm typically limit CuInSe2 film thickness to 5 xcexcm or greater. Such film thickness are a factor of 5-10 thicker than necessary to absorb incident sunlight, and result in high manufacturing materials costs. Researchers preparing CuInSe2 films by screen printing and sintering CuInSe2-based pastes report taking particular measures to avoid the formation of indium oxides deleterious to the CuInSe2 film properties. Screen-printed films are typically much thicker than required to absorb sunlight sufficiently. Screen printing and related film formation processes are unlikely to be economic unless effective strategies for forming powders with smaller particle diameters and for processing the powders to achieve good film quality are developed.
Various researchers have investigated small particles with median particle diameters of 100 nm and less as a pathway to preparing thin-film materials. Nanoparticles of a wide range of oxides (e.g. ZnO, SnO2, WO3, etc.) and chalcogenides (e.g. CdS, CdTe, etc.) have been reported, and thin films have been formed from nanoparticles by a variety of techniques. Such small particles can be deposited as particulate layers by a variety of processes including, for example, electrophoresis of colloidal suspensions and slurry spraying. CuInSe2 films have been prepared by spraying slurries of mixtures of single-phase, binary selenide nanoparticles such as CuxSe and InxSe, but film quality and device performance are poor due to insufficient interparticle diffusion. This implies that small median chalcogenide particle diameters alone do not provide improved particle-based thin film properties.
The use of an effective flux is known to be particularly important for promoting particle coalescence and grain growth in particle-based thin films. CdCl2 works well as a flux with Cd-based chalcogenide materials such as CdTe and Cd(Se,Te). A comparable flux has not been reported for CuInSe2 and related alloys. Se, CuCl, InCl3 and Cuxe2x80x94Se compounds have been evaluated as fluxes for screen-printed CuInSe2 layers, and non-negligible fluxing reportedly occurs only at relatively high temperatures in Cu-rich material when a liquid Cu2Se is present. Thus effective fluxing of CuInSe2 is possible only under conditions that result in Cu-rich CuInSe2 films unsuitable for solar cells, or in complex multi-step processes such as continuous co-evaporation in which the growing CuInSe2 film is temporarily made Cu-rich to effect transient fluxing via liquid Cu2Se before differentially adding In and Se to achieve an In-rich final film composition. Results are also poor when paste mixtures of Cu and In elemental powders are screen printed and overcoated with screen-printed Se powder paste.
Summarizing the prior art, coincident vapor phase processes are difficult to control on large areas; sequential vapor phase processes simplify some of the complexities, but composition uniformity and precursor phase segregation can be severe problems. Metal oxides are useful as phase-stabilized precursors, but uniform, high-rate, vapor phase deposition of oxide solid films is difficult, and spray pyrolysis of oxide solid films is inefficient due to poor materials use efficiencies. Particulate-based processes such as screen printing can have high materials efficiencies, but such processes generally work well only when efficient fluxing processes are available. A clear need exists for phase-stabilized precursors that can be easily converted to thin-film, photovoltaic materials. A need also exists for an easily controlled process for forming such precursors. It would be especially advantageous to provide a process using particulate precursors to fabricate thin films with well-controlled compositions on large areas, as well as materials and processing techniques for creating high quality, thin film products without contamination from residual flux materials.
The present invention provides unique methods for making phase-stabilized precursors in the form of fine particles, such as sub-micron multinary metal particles, and multi-phase mixed-metal particles comprising at least one metal oxide.
The invention further provides methods of using spraying and coating techniques to deposit thin, close-packed layers of multinary metal and/or mixed-metal metal oxide particles. In one embodiment slurry spraying is used to deposit layers of single-phase, mixed-metal oxide particulates from aqueous slurries sprayed in air on a heated substrate.
In one aspect of the invention, compound materials are formed using precursors comprising multi-phase particles comprising a metal oxide phase. In one embodiment, particulate precursors are deposited as layers on suitable substrates by efficient processes such as slurry spraying. The precursor layers are then converted to useful films by reacting precursor layer components together so as to cause interdiffusion, and/or by reacting the precursor layers with other reactant materials, such as overcoated layers, liquids, vapors or gases, causing ion exchange and interdiffusion. Of particular advantage are multi-phase particles in which each particle contains more than one compositional phase; multi-phase particulate precursors comprising metal oxides yield superior final film characteristics relative to single-phase metal oxide particulate precursors or precursors made up exclusively of non-oxide compound particles.
In another aspect of the invention, the precursors are multi-phase particles comprising both a metal oxide phase and a non-oxide phase. The presence of a non-oxide phase provides advantageous reaction pathways for converting particulate precursor materials to high-quality final materials by, for example, facilitating transient fluxing by intermediate phases during the conversion process. Of particular advantage are multi-phase particles comprising a metal oxide and a metal phase or binary compound phase in which the metal or compound phase facilitates fluxing and densification by forming liquid phases that facilitate transient fluxing between precursor particles during the conversion of the precursor to the final material. The differing conversion rates of the multiple phases of the precursor particles provides a pathway for transient fluxing due to a localized relative abundance of one or more constituents in a precursor material that has an overall deficiency of those constituents.
In another aspect of the invention, multinary metallic particles and mixtures of multinary metallic particles are used as precursors to further augment the advantages of particles comprising non-oxide phases, and to further simplify the conversion of precursor materials to desirable final materials. Of particular advantage are multinary, metallic particles that allow films of mixed-metal compounds to be prepared without the phase segregation problems typically associated with annealing solid mixed-metal layers.
In another aspect of the invention, there are provided methods for forming porous precursor layers intercalated with other materials, thereby further simplifying the conversion reactions of precursor layers to final films and further improving the density and electronic quality of the final films. Intercalation reduces the void space within a particulate-based layer and facilitates solid state reactions to form dense, coherent films. Intercalation provides pathways to facilitate transient localized fluxing in precursor layers that, through interdiffusion, reach efficient final film composition.
The present invention provides methods for making multi-phase mixed-metal particles comprising at least one metal oxide, and methods for making multinary metal particles. The invention teaches the use of slurry spraying, spray printing, spin coating and meniscus coating to deposit layers of particulate materials. The invention exploits the heretofore unrecognized advantages of using multi-phase particles as precursor materials for forming many desirable materials. In particular, the invention provides a route to thin-film materials using unique precursors such as metal oxide phases, non-oxide phases, and metallic phases. The invention further teaches that the utility of all kinds of particulate precursors can be augmented by intercalating the particles with other useful materials so as to facilitate low-temperature, solid-state ion exchange and densification. The full spectrum of unique advantages of this invention are more completely evident in the embodiments below.